Investigative writer Anthony Summers makes a compelling case for a new suspect
in the frame for the assassination of President John Kennedy in Dallas in
1963

Suite 850 of the Texas Hotel in Fort Worth. It is the morning of November 22 1963 and John Fitzgerald Kennedy, 35th President of the United States, is preparing for his short flight to Dallas.

Kennedy needs to woo Texas if he is to beat off Barry Goldwater, the Republican senator likely to be his opponent in the presidential race of 1964. But there are plenty of people in the Lone Star State happy to disappoint him, Right-wingers appalled by his apparent accommodation of Communist rule in Cuba and his administration's policy on civil rights.

Dallas is a hotbed of extreme conservatism, the only major city in America to have voted for Richard Nixon over Kennedy in 1960, and now here the president stands, with a copy of today’s Dallas Morning News. It carries a black-edged advertisement ''welcoming’’ the president, placed by a group calling itself the American Fact-Finding Committee. The ad consists of a list of accusatory questions: why is the Kennedy administration approved of by the US Communist Party? Why is the CIA being asked to arrange coups against Washington’s anti-Communist allies? And so on.

Jack Kennedy turns to his wife, Jackie. “We’re heading into nut country today,” he says. And then, pacing the room, he thinks aloud. “It would not be a very difficult job to shoot the President of the United States. All you’d have to do is get up in a high building with a high-powered rifle with a telescopic sight, and there’s nothing anybody could do.” Just over two hours later Kennedy is killed in the way he prophesied.

The shots echoing around Dealey Plaza that fine autumn day, scattering birds perched atop the Texas School Book Depository, resonate still. This Friday in Dallas, the 50th anniversary of the assassination will be marked with the tolling of church bells, a ceremonial flypast and readings of Kennedy speeches. America will remember its Camelot, and a milestone will have been reached.

“This is the moment when the Kennedy assassination tumbles over that dusty border dividing current affairs from history,” says Anthony Summers. He is standing in an outbuilding next to his riverside home in rural Co Waterford, Ireland, squeezed between shelves crammed with files marked “JFK”, the product of research that began in the 1970s and continued until this year.

Summers, who began his journalistic career on World in Action and later reported for Panorama, is author of widely praised books on Frank Sinatra, J Edgar Hoover and Marilyn Monroe. The Kennedy assassination was one of the first literary mountains he attempted to conquer, his efforts resulting in the 1980 book Conspiracy.

That book has now been reworked and republished with the title Not in Your Lifetime, a reference to an answer given by Earl Warren, chief justice of the United States, who headed the original inquiry. In 1964, when asked if all the information uncovered by his investigation would one day be made public, he replied: “Yes, there will come a time. But it might not be in your lifetime.”

Why – if, as the Warren Commission concluded, the chief executive of the US was murdered by an alienated misfit called Lee Harvey Oswald acting alone?

Summers has been trying to fill in the gaps created by this secrecy on and off for nearly 40 years. His work has led him to conclude that the US intelligence services concealed their connections with, or manipulations of, Oswald, and that there are reasonable grounds to suggest that Kennedy was the victim of a conspiracy, most likely involving the Mafia and Right-wing Cubans seeking the overthrow of Fidel Castro, both with axes to grind.

And now, he has a name, of a man who fits the bill as Kennedy’s killer, who is said to have confessed his involvement in the assassination before being killed in a commando-style raid on Cuba.

“I have tried to write a book for the sane citizen that is open both to the lone assassin theory and something more complex,” says Summers. “The evidence that Oswald fired is pretty strong but also full of weaknesses. Why, if it was Oswald alone, do 1,171 CIA documents on the subject remain classified in the year 2013? There is no doubt something was and still is being concealed.”

Summers initially avoided the story, regarding it as a graveyard of journalistic reputations. But then he was asked to prepare a Panorama report on the work of the US House Committee on Assassinations, which in 1979 concluded that Kennedy had “probably” been the victim of a plot, a second gunman being involved. “As I dug, I was struck by how poorly the story had been covered in its early days,” he remembers. “This was the post-Eisenhower era, and there was still trust in government.”

But what of Summers’s research? Who was this candidate for ''second trigger’’? The answer comes from an interview conducted in 2007 by Summers and Robert Blakey, formerly chief counsel to the House Committee on Assassinations, with 81-year-old Reinaldo Martinez Gomez, a Cuban exile living in Miami.

Martinez told them of a friend from student days called Herminio Diaz Garcia, an introverted but exceptionally brave man. Diaz, a crack shot, had worked as head of security at a casino in Havana run by the Mafia boss Santo Trafficante. He was also a political assassin, responsible for as many as 20 deaths, and at the time of the Kennedy assassination he was in the United States.

Martinez said that, while detained in one of Castro’s prisons, he had met Tony Cuesta, leader of an anti-Castro raid on Cuba in 1966 that had ended in Diaz’s death. Cuesta, who was badly wounded, told him about the night of the abortive raid and words uttered by Diaz that he would never forget. Said Martinez: “Herminio confessed to Tony Cuesta that he had taken part in the death of the US president.”

Years later, in Florida, Martinez heard the same from another source, an old friend and fellow Cuban exile, Remigio Arce. “Listen,” said Arce, “the one who killed the president was our little friend.”

Summers believes Martinez, who has since died, was telling the truth. “Martinez struck me – after two days of tough interviewing of a man in his eighties – as someone with nothing to gain, who appeared to be credible. Diaz ticks the boxes. He was a known political assassin, a marksman and had worked for Santo Trafficante, one of the two prime suspects in the assassination, together with fellow Mafia boss Carlos Marcello, named by the House Assassinations Committee. He was also involved with one of the more extreme anti-Castro groups.

“The Mafia had every motive to do away with President Kennedy because they were being pursued as never before by the government. At the same time, a large part of the anti-Castro movement felt betrayed by Kennedy. It [Diaz] is an important development that should be taken seriously.”

So where does Oswald fit in? “There is a subterfuge by the CIA in regard to Oswald’s visit to Mexico City a month before the assassination. Surveillance tapes that record Oswald’s visit seem to have been wilfully destroyed. I am absolutely not one of those who believe the CIA killed President Kennedy. But I am sure that they have hidden something.

“Oswald postured as a Left-wing, pro-Castro activist. The evidence suggests that, wittingly or unwittingly, he was being used by one of the US intelligence agencies as a low-level operative in the secret black propaganda war against Castro. Oswald had effectively declared himself a traitor when he defected to the Soviet Union and said he had offered information about his time on a U2 spy plane base, yet the CIA claims he was not questioned when he returned to the United States. That claim is not credible.”

It is possible, says Summers, that Oswald was saved from prosecution by agreeing to penetrate the pro-Castro movement.

“What may have happened is this: Oswald appears to be the assassin and is named in the media. If FBI and CIA were involved with Oswald in some capacity before the shooting – not in any way to do with the assassination – then the instinct would be to run for cover – ''For Christ’s sake, we’ve got to distance ourselves from this!’’ In covering that connection up, the agencies gave rise to the suspicion that they were concealing something much more sinister.”

He could go on – the botched autopsy in Washington, the connections of Jack Ruby, Oswald’s assassin, with the Mob, but discussion of the events surrounding that day in Dallas eats time. “So many aspects of the original investigation were messed up,” he says. “One would like to think there would be a better investigation of the shooting of a homeless man than there was in the killing of the President of the United States.”

The case refuses to go away, even after half a century. Summers, though, has had enough.

“After November 22 I don’t want to hear about the JFK case again. It is a nightmare to work on, not only a labyrinth but a labyrinth with lots of turnings off the labyrinth. The best answer is that we don’t know the answer, and probably never will.”

Where were you when Kennedy was shot? The Telegraph is inviting contributions from readers who can recall the momentous day in 1963 and their reaction to the news that united the world in shock. Please email readers@telegraph.co.uk. Selected contributions will be published on our website